The determination of analyte concentrations in physiological samples plays a prominent role in diagnosis and therapy of a variety of diseases. Analytes of interest include among others glucose, cholesterol, free fatty acids, triglycerides, proteins, ketones, phenylalanine, enzymes, antibodies, or peptides in blood, plasma, urine or saliva.
Typically, a physiological sample fluid, e. g. capillary blood, is applied to a test strip to evaluate the concentration of an analyte. The test strips are usually used in conjunction with a measuring device which measures some electrical properties, such as electrical current, if the strip is designed for detection of an electro-active compound, or for the measurement of light reflectance and/or transmittance, if the strip is designed for photometric detection. In systems with optical detection technology a mixture of enzymes and colour-generating materials known as chromogens is located on the test strip. The analyte contained in the physiological or aqueous fluid, which has been applied to the test strip, reacts with the reagents and causes a change in reflectance or transmittance thereby indicating the concentration of the analyte in the test sample.
For example, glucose is determined quantitatively by oxidizing glucose with glucose oxidase to gluconic acid. The reaction product hydrogen peroxide causes in conjunction with a peroxidase, such as horseradish peroxidase, the conversion of a substrate, i. e. an indicator, into a chromogenic product, which is detectable and relates proportional to the glucose concentration in the sample fluid.
Measuring the glucose concentration in samples of whole blood is a particularly common task. Since Diabetes causes dangerous physiological complications leading to the loss of vision, kidney failure and other serious medical consequences. Only a stringent therapy and disease management minimises the risk of these consequences with adjustments on exercise, diet, and medication. Some patients have to test their blood glucose concentration frequently with four or more measurements a day. These patients as well as clinicians and hospitals require an accurate, reliable, and ideally inexpensive method to adjust their treatment regimes to avoid the long-term complications of diabetes mellitus.
The increased awareness about diabetes, the acceptance of self-monitoring and self-treatment have been dependent upon the availability of suitable devices and let to the development of a multitude of devices and methods for personal use and point of care testing as well. Available are pregnancy, ovulations, blood coagulation, ketone and cholesterol tests, as example for a non-exhaustive selection, but most prominent in the area of self-monitoring is still the detection of glucose in capillary blood.
An exemplary device for monitoring the concentration of an analyte, e. g. glucose, in blood is disclosed in the U.S. Pat. No. 4,935,346. The method involves taking a reflectance reading from the surface of an inert porous matrix impregnated with a reagent that will interact with the analyte to produce a light-absorbing reaction product. Most of the devices of the prior art are designed to have one measurement area or measurement chamber in which the test sample is introduced directly or via a fluidic path or channel, the test chamber or test membrane contains all materials necessary for the reactions, which produce a detectable colour change of the sample fluid.
U.S. Pat. No. 5,430,542 discloses a disposable optical cuvette and method of manufacturing. The cuvette comprises two optically transparent liquid impermeable plastic sheets. A third adhesive sheet is positioned between the two transparent plastic sheets and all three sheets are pressed and sealed together.
U.S. Pat. No. 5,268,146 discloses a qualitative test panel for testing a sample for the presence of an analyte containing all reagents and components necessary to achieve a visible indication of the presence or absence of an analyte in the sample.
U.S. Pat. Nos. 4,761,381 and 5,997,817 disclose devices wherein the liquid samples to be analysed are applied to sample application ports which give the liquid entry to capillary channels leading to reaction chambers which contain material capable of detecting the components of interest in the liquids.
U.S. patent application Publications US 2002/0110486A1 and US 2003/0031594 A1 disclose fluidic medical diagnostic devices permitting the measurement of analyte concentration or a property of a biological fluid, particularly the coagulation time of blood, the devices having at one end a sample port for introducing a sample and at the other end a bladder for drawing the sample via a channel to a measurement area, in which a physical parameter of the sample is measured and related to the analyte concentration or property of the fluid.
Due to raw material and process variations in large-scale manufacture of these strips an adequate strip-to-strip reproducibility from one batch to the next is not guaranteed. Therefore, it is necessary to assign a calibration code to each lot of strips that corrects for this vanability. The calibration code may be marked on the strip container, and the user must enter the code into the meter when a new batch of strips is used. If the user fails to enter a new calibration code or enters an incorrect one, the resulting measurement will be incorrect. Some prior art strips, e. g. the strip disclosed in U.S. Pat. No. 6,168,957B1, are designed to incorporate the calibration code on the strip, thus the meter can read the calibration code before calculating the glucose concentration. The disposable nature of single use diagnostic strips allow only destructive testing, due to the consumption of reagents during the determination step, and thus permit only a statistical evaluation of the batch performance by the manufacturer, which does not give 100% certainty of the performance of an individual test strip.
More importantly, these types of calibration codes convey only retrospective information to the analytical strip-reading device or meter. Thus, a meter cannot assess the true history of a particular reagent test strip, e. g. incorrect storage conditions or faulty packaging, and will generate an error message only if the strip provides completely erroneous and off scale readings in comparison to the pre-program data or validation methods.
The user can only check and proof the accuracy and functionality of a reagent test strip with specially prepared control solutions of known concentrations provided by the manufacturer. Nevertheless, this method is also disadvantageous, since the quality check leads to increased strip consumption and therefore to increased costs. Likewise, this method does not take into consideration the quality variations within a batch.
Some of the devices of the prior art have integrated positive and/or negative controls, which are activated by the addition of the sample. For instance, in the above mentioned U.S. Pat. No. 5,268,146 preferred embodiments of the test device include a built-in positive control and/or a built-in negative control which consist of further measuring areas containing reagents which will either induce the visible change in the indicator by themselves or prevent the change from occurring independently of the presence or absence of the analyte in the test sample. Also, the test device of the U.S. Pat. No. 4,578,358 for detecting the presence of occult blood in bodily substances includes positive and negative control areas.
An integrated positive or negative control as disclosed in the above two patents and known commonly from pregnancy tests provides only useful information in conjunction with qualitative and threshold test panels or strips indicating the presence or absence of an analyte but is meaningless for the quality assurance of quantitative determination of analytes such as glucose in whole blood.
Furthermore, the measuring procedure may be impaired by other variable factors in the physiological sample fluid. A typical complication in whole blood analysis is the variability of erythrocyte levels, leading to results which may not reflect the real analyte concentration of the sample.
In view of the aforementioned shortcomings, it is the object of the present invention to provide a device which has an integrated calibration system, which accounts for and compensates any variability may it be generated by fluctuations in the production process or by the variability of the analysed sample itself to assure the user that the test has been properly performed and the result is accurate and reliable.
So far, no dry reagent test strip with integrated calibration system has been disclosed by prior art, but a variety of prior art publications describes test strips with pluralities of reactions zones used to detect a plurality of analytes or to integrate positive or negative controls as indicated above.
A particular interesting prior art test strip comprising a plurality of reaction zones utilised for quality assurance purposes but not for a strip internal calibration procedure has been disclosed in U.S. patent application Publications US 2002/0110486A1 and US 2003/0031594 A1. The test strip requires a volume of about 20 μL blood and is used to determine the prothrombin time, an important parameter to characterise blood coagulation. However, if a user has to test several times a day, as required for proper management of diabetes mellitus, these large sample volumes are unpractical and disadvantageous especially in comparison with the state of the art blood glucose systems which require only about 1 μL of whole blood but require in all events a patient performed calibration procedure as well.
A reduction of the volume of the channels and cavities forming the measuring cavities in the described strip would require complex and expensive production procedures, such as “micro-moulding”, which are less suitable for large-scale production of inexpensive and disposable sensors.
Accordingly, it is a further object of the present invention to provide an analyte test system for dry reagent test strips, which requires not only small volumes of physiological or aqueous fluid but also a production process which does not involve many and complicated production steps and therefore is inexpensive and usable for products assisting patients in self-monitoring blood glucose or other important physiological parameters.